1. Field of the Invention
This invention relates to an auxiliary air amount control device for an internal-combustion engine, and particularly to an auxiliary air amount control device for an internal-combustion engine, which for the purpose of permitting uninterrupted control of the number of revolutions of the engine mainly during an idling operation of the engine, enables an auxiliary air amount control valve disposed in a bypass interconnecting the upstream and downstream sides of a throttle valve to be controlled in such a manner that the idling operation will be carried out even when an arithmetic device serving to set the opening degree or the position of the control valve is out of order.
2. Description of the Prior Art
Heretofore, it has been customary, while the internal-combustion engine is in the process of an idling operation, namely an operation continued with the throttle valve of the engine kept in a practically closed state, to control the number of revolutions of the engine by the use of an auxiliary air amount control valve (hereinafter referred to simply as a "control valve") disposed in a bypass interconnecting the upstream and downstream sides of the throttle valve. In the auxiliary air amount control device of this principle, the opening degree or the position of the control valve is feedback controlled such that the actual number of revolutions of the engine will agree with the target number of idling revolutions set in advance as a function of the temperature of engine cooling water. Such a feedback control method is described in detail in the specification of Japanese patent application No. SHO 60(1985)-137,445, for example.
For use in combination with said system for feedback control of the number of revolutions of the engine, another auxiliary air amount control device has been proposed which is adapted to detect the real electric current flowing to a drive coil of the control valve and effect feedback control of the electric current flowing to the drive coil so that the magnitude of the real electric current will agree with the target magnitude of electric current obtained in advance by dint of the system for the feedback control of the number of revolutions of the engine mentioned above. The method which controls the number of revolutions of the engine during an idling operation thereof by utilizing the system for the feedback control of the number of revolutions of the engine and the system for feedback control of the electric current is described in detail in the specification of Japanese patent application SHO No. 60(1985)-233,355, for example.
The conventional techniques described above have entailed the following problems.
The feedback control by either of the methods described above is effected by an electronic control device disposed in an automobile carrying the engine of interest. If the arithmetic unit of electronic control device develops trouble for one reason or another, the feedback control will no longer be normally carried out. As the result, there ensues a possibility that the idling operation will no longer be continued stably.
There is another possibility that the value of the command to be fed to the control valve or the opening degree of the control valve will fall to nil. There are times when the idling operation will become substantially impossible.